High-temperature thermal behaviors of XH₂PO₄ (X = Cs, Rb, K, Na) and LiH₂PO₃

XH₂PO₄ ionic compounds have emerged as a viable electrolyte for intermediate temperature fuel cells, and here have been subjected to thermal analysis to clarify their high-temperature properties. Thermoanalytical peaks were identified at 231.5, 239 and 349 °C for CsH₂PO₄; 127, 250 and 354 °C for RbH₂PO₄; 232, 270 and 319 °C for KH₂PO₄; 223, 330 and 352 °C for NaH₂PO₄; also, 195 and 220 °C for LiH₂PO₃ (peak temperature values as measured at the same heating rate of 10 K/min). The thermal events at 231.5 °C in CsH₂PO₄ and 127 °C in RbH₂PO₄ were previously interpreted as thermal decomposition by numerous researchers, but we confirm their origin in structural phase transition. The high-temperature variations in KH₂PO₄ and NaH₂PO₄ are entirely due to thermal dehydration rather than phase transition. We have also examined LiH₂PO₃, and found, for the first time, an endothermic peak at 195 °C, and attributed it to structural phase transition.